Ken,
Thanks for all of the help.
I just stuffed your number in the ARRL mast calculator.
I will have an Optibeam 11-5 one foot above the top (15.7 sqft - 71ft),
the Moxon 11 feet above that (7 sqft - 82ft) plus 5 sqft of mast and a
3 sqft 6m beam at the top.
I get mast survival, three inch, 0.25 wall, 87Kpsi material, of 130 mph.
Over 900lbs of wind force. Looks like the thrust bearing is pretty
important.
The tower will take that load to 110 mph, the antennas start coming
apart at 100 mph.
So...when the F1 tornado comes dancing through the neighborhood the
tower will stay up, the mast won't bend and every roof for 100 yards
radius will be gone.
That mast calculator really opened my eyes. Everybody using two inch
masts needs to see just how overloaded they may be. My proposed
antenna's on a two inch mast of 87K 1026 DOM material fails at 88 mph.
Switching to 4130 and 110K material gets you another 11 mph to 99.
Diameter is everything.
The calculator is fun to play with.
On 4/20/2016 1:58 AM, Ken K6MR wrote:
A week ago or so AC0H posted a simple question: how much wind load for a W6NL 40M
Moxon yagi. As I had just built one and installed it (but not bothered to do the
mechanical model) I thought I’d dive in.
I posted some values from Yagi Stress, and a good conversation got going on how to calculate
the maximum wind load. Some numbers were posited but I still wasn’t sure. Even though
I’ve used Yagi Stress a fair amount, I was not quite sure how it did what it did.
VE7RF and VE1DT pointed us to an old posting by K7NV regarding a modern wind load calculation model
published in Communications Quarterly. Thanks to AC6LA who found the article and posted a link to
the article and K7NVs posting. In essence, I found out “you’re doing it wrong”.
After reading through the article and (I think) understanding the concepts I
ran through the calcs again. Needless to say, the results are different.
In summary (assuming I did it all correctly), the antenna has a maximum wind
load of 6.22 sq. ft.. This occurs with the boom broadside to the wind. The
model includes RG8 coax to the DE and the balun box is included. For
comparison, the wind load with the antenna pointed into the wind (so loads from
the elements and standard Cushcraft mounting channels) is 5.98 sq. ft..
According to the K5IU model, the maximum wind load of the antenna is the larger
of these two values.
I also used Yagi Stress to calculate a torque compensator plate. Because of the coax and
balun there is a small value of rotating torque when broadside. Allowing for the torque
development by the different sizes of the element tees, the plate is pretty small. But it
does raise the total broadside area to 6.42 sq. ft. So I believe we can safely say
“under 7 sq. ft.” is a pretty safe value.
The element and boom sizes I used were assuming converting an XM240 and using the latest (2012)
design that is labeled “W2SC 100 mi/h”. This is the one that puts new 1.5”
center sections in the elements.
I’ve posted the summary I wrote up if you are interested in the numbers. And please
let me know if there is some bone-head math error or if I really don’t understand the
concepts. The original XM240 spec is 5.5 sq. ft., so the new values seem reasonable.
http://bit.ly/1qYohrZ
Thanks to everyone who posted info on this subject. The best part about this
hobby (for me) is learning new stuff. This was definitely new.
Ken K6MR
--
R. Kevin Stover
AC0H
ARRL
FISTS #11993
SKCC #215
NAQCC #3441
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